Recursos Marinos y Pesquerías

Oil spill effects on subtidal seabeds assessed by scuba divers [Poster]

07/07/05 ,

Pablo Pita & Juan Freire. Preliminary analyses of the status of the Galician subtidal
seabed after the Prestige oil spill assessed through direct observations by
scuba divers. Poster (jpg). Working paper (pdf).

VERTIMAR 2005 – Symposium on marine accidental oil spills. 13 – 16 july 2005, Universidade de Vigo. Vigo (Spain).

Paper also available at FEDAS website

INTRODUCTION
On the 13th of November 2002, the
oil–tanker Prestige, with a cargo of 77000 tons of oil, sunk 120 miles off
Islas Cies. During the shipwreck of the tanker, outstanding political and
social circumstances took place which have greatly influenced the effects
derived from the accident, bringing on the uninterrupted emission of oil in
consecutive spills in an unusually large scale in space and time. The
Federación Gallega de Actividades Subacuáticas (FEGAS) [Galician Federation of
Recreative Scuba Diving], a private organisation concerned with the integral
protection of the Galician marine environment, undertook fieldwork to establish
the initial degree of pollution by hydrocarbons in the seabed in a series of
coastal areas at a time when the the mismanagement of the authorities
highlighted the need and value of private iniciatives. A total of 50 scuba and
free divers, all of them voluntaries and coordinated by FEGAS, made a
considerable sampling effort including 4500 m of underwater transects, covering
a sampling area of 25500 m² (see fig.1) of seabed prospected during
the 9 months (10/12/2002 to 5/08/2003).

METHODOLOGY
The Galician coast was divided in sections under
the supervision of one of the diving clubs affiliated to FEGAS which was in
charge of collecting the information generated by the divers. Each diver
prospected a transect during 10 minutes at a constant depth (see fig. 2),
collecting standardised data about observed macroscopic pollution (fig. 3).
Observations of oil were aproximated to simple geometric forms to allow the
estimation of the total volume of hydrocarbons observed. These estimates used
values of 0.99 g.cm^-3provided by CSIC (2003). Sampling areas were calculated using a band
of variable width for each transect, depending of the diver´s estimated
visibility of the diver and an average length of 75 m by transect.

RESULTS
For all the sampling stations, the
total oil found was 6064.4 kg. This quantity, estimating an average visibility
of 3.15 m, would suppose 0.80 kg·m^-2 both in the water column and
the seabed of the sampling stations in which the presence of oil was
noticed. It could be concluded that the
most affected area corresponded to the Parque Nacional Marítimo-Terrestre de
las Islas Atlánticas de Galicia. The quantitative results obtained in the Isla
de Sálvora were especially high (see fig. 4). The dives performed in Costa da
Morte showed high amounts of oil in the water column, which agrees with several
pieces of information which point out that this area was the most affected coastal area (fig. 4). Considerable
quantities of oil have been obtained in the Caldebarcos beach, near Carnota (A
Coruña), and in the Rías de Ferrol and Aldán (fig. 4). Among the organisms
affected by the acute phase of oil spill, we should highlight the high number
of oiled specimens of the spider crab Maja
brachydactyla observed during the various dives (fig. 5) and considerable
extents of affected Zostera marina
beds, a habitat protected by the current legislation (fig. 2).